This objective is to investigate the capabilities of new quantitative respirator fit test method that seems to be less complex, more portable and more cost-effective than current methodologies. The new method is based on exhausting air from a temporarily sealed mask, thereby generating a negative pressure that replicates the inspiratory driving force for leakage into the mask. The exhaust flow rate that is required to generate and sustain a pre-selected negative pressure in the mask is a direct measure of the mask leakage flow rate, which can be used as an index of respirator fit. Specific aims include refining and automating portions of negative pressure fit test system used in a preliminary exploration of the new method. Sensitivity and precision limits of the new method will be determined through comparison with an aerosol fit test system based on corn oil. A test population recruited from Air Force personnel already assigned to a respiratory protection program will be used and anthropometrically compared to a defined respirator test panel. The capability of the new method to measure respirator fit representative of non-rest work rates will be examined by using a computerized open-circuit metabolic technique. Field measurements of respirator fit will also be attempted with the new method.